This invention relates to a method of making more efficiently p-doped silicon films with higher acceptor concentrations and devices made therefrom so that improved p-n and p-i-n devices can be now produced in a batch or continuous process involving the successive deposition and formation of all or partially amorphous p and n type silicon films. While the invention has utility in making diodes, switches, and amplifier devices like transistors, it has its most important application in the making of photoconductive devices like solar cells or other energy conversion devices.
When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor devices manufacturing industry. This was due to the ability of scientists to grow substantially defect-free germanium and particularly silicon crystals, and then turn them into extrinsic materials with p-type and n-type conductivity regions therein. This was accomplished by diffusing into such crystalline material parts per million of donor (n) or acceptor (p) dopant materials introduced as substitutional impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type. The fabrication processes for making p-n junction and photoconductive crystals involve extremely complex, time consuming, and expensive procedures. Thus, these crystalline materials useful in solar cells and current control devices are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, and when p-n junctions are required, by doping such single crystals with extremely small and critical amounts of dopants.
These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an impassable economic barrier to the large scale use of crystalline semiconductor solar cells for energy conversion. Further, crystalline silicon has an indirect optical edge, which results in poor light absorption in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the crystalline material is replaced by polycrystalline silicon with cheaper production processes, the indirect optical edge is still maintained; hence the material thickness is not reduced. The polycrystalline material also involves the addition of grain boundaries and other problem defects. On the other hand, amorphous silicon has a direct optical edge and only one-micron-thick material is necessary to absorb the same amount of sunlight as crystalline silicon.
Accordingly, a considerable effort has been made to develop processes for readilly depositing amorphous semiconductor films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be readily doped to form p-type and n-type materials where p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short diffusion lengths, making such films unsuitable for solar cell applications. Additionally, such film cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making Schottky barrier or p-n junctions for solar cells and current control device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon and germanium, W. E. Spear and P. G. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on "Substitutional Doping of Amorphous Silicon", as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196 (1975), toward the end of reducing the localized states in the energy gap in the amorphous silicon or germanium to make the same approximate more closely intrinsic crystalline silicon or germanium and of substitutionally doping said amorphous materials with suitable classic dopants, as in doping crystalline materials, to make them extrinsic and of p or n conduction types. The reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein a gas or silane (SiH.sub.4) was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and deposited on a substrate at a substrate temperature of about 500.degree.-600.degree. K. (227.degree. C-327.degree. C.). The material so deposited on the substrate was an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material, phosphine (PH.sub.3) gas for n-type conduction or diborane (B.sub.2 H.sub.6) gas for p-type conduction were premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used were between about 5.times.0.sup.-6 and 10.sup.-2 parts per volume. The material so deposited included supposedly substitutional phosphorous or boron dopant and was shown to be extrinsic and of n or p conduction type. However, the doping efficiency for the same amount of added dopant material was much poorer than that of crystalline silicon. The electrical conductivity for highly doped n or p material was low, being about 10.sup.-2 or 10.sup.-3 (.OMEGA.cm).sup.-1. In addition, the band gap was narrowed due to the addition of the dopant materials, especially in the case of p-doping using diborane. These results indicate that diborane did not efficiently dope the amorphous silicon but created localized states in the band gap.
As expressed above, amorphous silicon, and also germanium, is normally four-fold coordinated, and normally has microvoids and dangling bonds or other defective configurations, producing localized states in the energy gap. While it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to decrease substantially the density of the localized states in the energy gap, toward the end of making amorphous material approximate more nearly the corresponding crystalline material.
However, the incorporation of hydrogen not only has limitations based upon the fixed ratio of hydrogen to silicon in silane, but, most importantly, various Si:H bonding configurations introduce new antibonding states which can have deleterious consequences in these materials. Therefore, there are basic limitations in reducing the density of localized states in these materials which are particularly harmful in terms of effective p as well as n doping. The resulting unacceptable density of states of the silane-deposited materials leads to a narrow depletion width, which in turn limits the efficiencies of solar cells and other devices whose operation depends on the drift of free carriers. The method of making these materials by the use of only silicon and hydrogen also results in a high density of surface states which affects all the above parameters.
After the development of the glow discharge deposition of silicon from silane gas was carried out, work was done on the sputter depositing of amorphous silicon films in the atmosphere of a mixture of argon (required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon film. This research indicated that the hydrogen acted as an altering agent which bonded in such a way as to reduce the localized states in the energy gap. However, the degree to which the localized states of the energy gap were reduced in the sputter deposition process was much less than that achieved by the silane deposition process described above. The above described p and n dopant gases also were introduced in the sputtering process to produce p and n doped materials. These materials had a lower doping efficiency than the materials produced in the glow discharge process. Neither process produced efficient p-doped materials with sufficiently high acceptor concentrations for producing commercial p-n or p-i-n junction devices. The n-doping efficiency was below desirable acceptable commercial levels and the p-doping was particularly undesirable since it reduced the width of the band gap and increased the numbers of localized states in the band gap.
The prior art amorphous silicon, which has been altered by hydrogen from the silane gas in an attempt to make it more closely resemble crystalline silicon and which has been doped in a manner like that of doping crystalline silicon, has characteristics which in all important respects are inferior to those of doped crystalline silicon. Thus, inadequate doping efficiencies and conductivity were achieved especially in the p-type material, and the photoconductive and photovoltaic qualities of these silicon films left much to be desired.
A substantive breakthrough in forming amorphous silicon films with a very low density of states was achieved by the inventions disclosed in U.S. Pat. Nos. 4,217,374 and 4,226,898 issued 8/12/80 and 10/7/80, respectively, to Onshinsky, et al which produced amorphous films, particularly silicon amorphous films, having the relative favorable attributes of crystalline semiconductor materials. (The former patent discloses the deposition of improved amorphous silicon films using vapor deposition thereof and the latter patent discloses the deposition of improved amorphous silicon films by glow discharge of silicon-containing gases.) The improved amorphous intrinsic silicon films produced by the processes disclosed therein have reduced number of states in the band gap in the intrinsic material and provide for greatly increased n-doping efficiencies, high photoconductivity and increased mobility, long diffusion length of the carriers, and low dark intrinsic electrical conductivity as desired in photovoltaic cells. Thus, such amorphous semiconductor films can be useful in making more efficient devices, such as solar cells and current controlling devices including p-n junction devices, diodes, transistors and the like.
The invention which achieved these results incorporates into the amorphous films, preferably as they are being deposited, altering or compensating materials which are believed to form alloys with the amorphous semiconductor materials and modify the same so as to greatly reduce the localized states in the energy gap thereof to make the same equivalent in many respects to intrinsic crystalline silicon. In the process of forming silicon films disclosed in said U.S. Pat. No. 4,226,898 a compound including silicon as an element thereof is decomposed by glow discharge decomposition to deposit amorphous silicon on a substrate along with the incorporation of a plurality of alterant elements, preferably activated fluorine and hydrogen, during the glow discharge deposition.
In these specific embodiments of the invention disclosed in the latter application, silicon is deposited in a batch mode at a substrate temperature of about 380.degree. C. by the glow discharge of silicon tetrafluoride (SiF.sub.4) which supplies the silicon in the deposited amorphous films and fluorine as one altering or compensating element. While silicon tetrafluoride can form a plasma in a glow discharge, it is not by itself most effective as a starting material for glow discharge deposition of silicon. The atmosphere for the glow discharge is made reactive by adding a gas like molecular hydrogen (H.sub.2), which is made reactive by the glow discharge by changing it to atomic hydrogen or hydrogen ions or the like. This reactive hydrogen reacts in the glow discharge with the silicon tetrafluoride so as to more readily cause decomposition thereof and to deposit amorphous silicon therefrom on the substrate. At the same time, fluorine and various silicon subfluorides are released and made reactive by the glow discharge. The reactive hydrogen and the reactive fluorine species are incorporated in the amorphous silicon host matrix as it is being deposited and create a new intrinsic material which has a low number of defect states. A simple way to consider the new alloy is that there is a satiation or capping or dangling bonds and the elimination of other defects. Hence, these alterant elements reduce substantially the density of the localized states in the energy gap, with the foregoing beneficial results accruing.
When it is desired to provide n-type and p-type conduction in the amorphous silicon semiconductor matrix, the latter patent recommends incorporation of modifier elements in gaseous form during the glow deposition of the film. The recommended modifier elements or dopants for n-type conduction are phosphorus and arsenic in the form of the gases phosphine (PH.sub.3) and arsine (AsH.sub.3). The recommended modifier elements or dopants for p-type conduction are boron, aluminum, gallium, and indium, in the form of the gases diborane (B.sub.2 H.sub.6), Al(C.sub.2 H.sub.5).sub.3, Ga(CH.sub.3).sub.3, and In(CH.sub.3).sub.3. The modifier elements were added under the same deposition conditions as described for the intrinsic material with a substrate temperature of about 380.degree. C.
While the process for making deposited silicon devices in the aforesaid applications represents a significant improvement, making possible the production of improved solar cells and other devices, the p-doped deposited silicon material did not have a p-type conductivity as efficient as desired. As reported in the Journal of Non-Crystalline Solids, Volumes 35 and 36, Part I, January/February, 1980, pp 171-181, with the addition of 500 ppm PH.sub.3 in the deposition gases, corresponding to an n.sup.+ to layer, no band gap change is evident between the n.sup.+ layer and the intrinsic material. With the addition of diborane (B.sub.2 H.sub.6) in the deposition gases, significant changes in optical absorption takes place. The implication is that a new alloy involving boron has been synthesized which possesses a more narrow band gap and exhibits p-type characteristics. It is possible that three-center bonds unique to boron are responsible in part for this behavior. This is in contrast to the results obtained when phosphorous or arsenic are added where a conventional n-type material is produced.
While devices like a Schottky barrier or MIS device can be made with or without p-doped films, they are difficult to manufacture since the properties of the thin barrier layer commonly used therein are difficult to control and frequently the thin layer cannot be efficiently encapsulated to prevent diffusion of environmental elements therethrough, with the result that the device is frequently unstable. In addition, such structures lead to a high sheet resistance in the upper level of the device. It appears that a photovoltaic cell having desired efficiency and stability requires utilizing a p-n or p-i-n junction. For this purpose, an improved p-doped material is desirable to increase the efficiency of the cell.
In making the fluorine and hydrogen compensated glow discharge deposited silicon films disclosed in the latter aforesaid application, the silicon is preferably deposited at a substrate temperature of about 380.degree. C. Above this substrate temperature, the efficiency of the hydrogen compensation gradually decreases and at temperatures above about 450.degree. C. reduces significantly, because the hydrogen does not readily combine with the depositing silicon at such temperatures.
As noted above, it has been discovered that the introduction of the gaseous p-dopant materials, while producing a p-type material, do not produce a material with a p-type conduction efficiency as would be theorized if only the desired four-sided or tetrahedral bonding were taking place. It appears that at the glow discharge substrate temperatures of 400.degree. C. or below, which are necessary for the most efficient hydrogen compensation of the silicon material, some of the would-be p-dopant materials are threefold rather than tetrahedrally coordinated, because of the absence of crystalline constraints, thus leading to additional states in the gap and no doping. Other processes involving diborane lead to the formation of three-centered bonds, or other less efficient combinations because the metallic or boron parts thereof do not readily disassociate completely from their hydrocarbon or hydrogen companion substitutents and so do not in such form provide an efficient p-doping element in the silicon host matrix. Furthermore, states are added in the band gap of such materials which are believed to reduce the p-doping efficiency achieved.
Therefore, appreciable effort has been made to improve the p-doping efficiency of said p-doping elements in glow discharge deposited silicon material. Glow discharge deposition of silicon for photovoltaic and other applications requiring intrinsic layers or p-n junction formed depletion regions presently appears to be the preferred deposition method therefor, since the degree of hydrogen and fluorine compensation and reduced density of states in the resulting material are superior to that obtained by vapor deposition or sputtering of silicon.
The present invention has to do with a method of more efficiently p-doping material in a glow discharge silicon deposition batch or continuous process to produce more efficiently doped p-type materials and p-n and p-i-n junction devices incorporating the more efficiently p-doped silicon materials. The methods of making p-doped material in the prior art have been limited to use of conventional dopant gases, such as diborane, under the deposition conditions optimized for the intrinsic materials. No one heretofore considered p-dopant gaseous boride compounds (such as B.sub.2 H.sub.6) and p-dopant metal gaseous compounds useful in glow discharge deposition of amorphous (or polycrystalline) silicon deposited at substrate temperatures above about 450.degree. C., which have been considered to be outside the temperature range required for the preparation of the useful amorphous silicon.
The present invention also encompasses the method of making a more efficiently p-doped glow discharge deposited silicon by depositing the material above about 450.degree. C. The loss of the advantages of hydrogen compensation in the silicon materials deposited at these high temperatures is more than overcome by the increased efficiency by the p-doping achieved, especially where the p-doped deposited layer is to form an ohmic p.sup.+ interface with the associated electrode. As previously stated, it appears that at these high temperatures the boron or metal p-dopant elements are so substantially disassociated from the hydrogen and hydrocarbon elements of the gaseous compound used that the three center or other undesirable bonding configurations are eliminated. The desired four-sided (tetrahedral) bonding which is efficient for p-doping is thus obtained. Although p-dopant metal (i.e. Al, Ga, In, Zn, and Tl) compound gases were also not effective as p-type dopants in the glow discharge deposition of silicon using substrate temperatures at or below about 400.degree. C., these elements are good p-dopants in gaseous compound form using the much higher silicon glow discharge substrate temperatures described herein (that is temperatures at least about 450.degree. C.). It should be noted that although the high substrate temperatures above about 450.degree. C. can result in inefficient hydrogen compensation of the silicon material, the material is still effectively fluorine compensated since fluorine efficiently combines with the deposited silicon at substrate temperature up to the range of 700.degree. C. to 800.degree. C.
For amorphous silicon deposited without hydrogen or fluorine compensation, the crystallization process becomes inportant at substrate temperatures of about 550.degree. C. For depositing amorphous silicon with hydrogen compensation and/or alloying, the amorphous state substantially is maintained up to substrate temperatures of about 650.degree. C. For amorphous silicon compensated with hydrogen and doped with boron, the amorphous state remains to substrate temperatures about 700.degree. C. The addition of fluorine such as in the materials of this invention, extends the amorphous state of the deposited material to still higher substrate temperatures. From this it is clear that the present process produces fluorine compensated amorphous silicon doped with boron at substrate temperatures about 700.degree. C. Doping levels achieved with deposition substrate temperatures such that the hydrogen and fluorine compensated silicon films remains substantially amorphous, will be sufficient for certain doping applications. For still higher doping levels, higher deposition substrate temperatures may be used such that the amorphous material will become mixed with crystallites of silicon, or become substantially polycrystalline.
The inclusion of crystallite material into the amorphous deposited silicon or the use of substantially polycrystalline p-doped material does not impair the efficiency of a p-n or p.sup.+ -i-n.sup.+ photovoltaic device. The efficiency is not impaired because the efficiency of p doping in polycrystalline silicon is well known, and because the optical absorption of the crystallites will be lower than that of the amorphous material, so the photon absorption in the photoactive layer will not be affected. For amorphous materials with high absorption coefficients, the p.sup.+ layer in a p.sup.+ -i-n.sup.+ structure is kept as thin as possible, less than 1000 angstroms, to minimize absorption of photons since it is a non-photoactive layer. The layer thickness still provides enough positive carriers to bend the conduction and valence bands between the p.sup.+ and the intrinsic layer in the device for efficient photovoltaic action. The admixture of silicon crystallites into the amorphous silicon, not only does not impair the efficiency of a p.sup.+ -i-n.sup.+ device, but also may assist the efficiency of a p-n photovoltaic device because of the increased hole mobility and increased photoconductivity of the crystalline p material compared with amorphous p material.
The present invention also discloses the method of eliminating the difficulty of p-doping by utilizing an unconventional non-gaseous material as a dopant. The method includes heating a solid metal to a high temperature to evaporate the metal and then feed the metal vapor directly into the glow discharge chamber with the silicon deposition gases continuously or intermittently. The p-dopant metals in a vaporized metallic form are effective in the glow discharge deposition of silicon at lower substrate temperatures, where fluorine and hydrogen compensation is desired. These evaporated p-dopant metals can also be utilized with glow discharge silicon deposited film at higher substrate temperatures where hydrogen compensation is not needed.
Utilizing the present invention, p-dopant boron and metal materials may be deposited in a continuous process combined with n and intrinsic type glow discharge deposited amorphous materials to manufacture improved p-n and p-i-n junction photovoltaic and the like devices. In the continuous process, the materials are glow discharge deposited upon a web substrate as it is continuously or stepwise moved through separate deposition stations each having the substrate temperature and other environmental conditions necessary to efficiently deposit the particular desired p and n and/or intrinsic type silicon films on the continuous web. In the continuous manufacturing process of the invention, each deposition station is dedicated to depositing one layer (p, i, or n), because the deposition materials contaminate the station background environment and are not easily removed.
While the principles of this invention apply to the aforementioned amorphous and polycrystalline type silicon semiconductor materials, for purposes of illustration herein and as setting forth preferred embodiments of this invention, specific reference is made to gaseous boron and gaseous and evaporated metal p-dopant material glow discharge deposited with the silicon material at substrate temperatures of between 450.degree. C. to 700.degree. C. The deposited film will be fluoride compensated throughout the substrate temperature range, but the hydrogen compensation will decrease with increasing substrate temperature. Also, the evaporated metal p-dopant materials may be glow discharge deposited with the silicon material at substrate temperatures below 400.degree. C. to form a hydrogen and fluoride compensated p-doped material.
In summary, to bring the significance of the present invention into focus, it is believed that the present invention enables the fabrication of more efficient p-type amorphous semiconductor films for use in the manufacture of solar cells and current devices including p-n and p-i-n devices. Additionally, the present invention provides for viable mass production of the various devices in a glow discharge environment with boron or at least one of the metals Al, Ga, In, Zn, or Tl providing the p-dopant material at prescribed substrate temperatures.
The above-described and other advantages and features of the present invention will become more apparent upon making reference to the following additional disclosures, the drawings and the claims.